1. Field of the Invention
The present invention relates to a helical scan type video tape recorder (hereinafter called VTR), and more particularly, to a VTR wherein a sag distortion, a skew distortion and a phase distortion are eliminated from a reproduced video signal, thereby providing an improved picture quality.
2. Description of the Prior Art
Heretofore, a sag distortion has been encountered in a reproduction of video signal by a signal head VTR. FIG. 1 is a block diagram showing a prior art video signal reproducing system and FIG. 2 is a plan view of a rotary drum having a single video head to be used therein.
Referring to FIG. 1, the prior art video signal reproducing system comprises a rotary transformer 11, an amplifier 12, a signal processing circuit 23, a demodulator circuit 20, a clamp circuit 21, a synchronous separation circuit 22, a clamp circuit 45, an A/D converter 24, a memory unit 26, a memory unit control circuit 31, and a D/A converter 33. In FIG. 2, 3 denotes a rotary drum, 5 denotes a single video head and 2 denotes a video tape.
In the prior art VTR shown partly in FIG. 1, the single video head 5 reproduces a FM signal 13 shown in FIG. 3(d) from the video tape 2 by being switched (switcher is not shown) by a head switching signal 17 shown in FIG. 3(b). The reproduced FM signal 13 reproduced by the single video head 5 is demodulated at the demodulator circuit 20 into a video signal 43 having a signal field and a no-signal field, alternatively, as it is shown in FIG. 3(e). This video signal 43 is then fed to the clamp circuit 21 before applying to the synchronous separation circuit 22 for fixing a sync-tip level to a constant level. However, a time constant of the clamp circuit 21 has normally been selected to be 10 times as much as an interval of horizontal synchronizing signal, thereby generating sags in a clamped video signal 44 as shown in FIG. 3(f). Therefore, if such a clamped video signal 44 is fed to the A/D converter 24 for converting it into a digital signal and further a no-signal field thereof is supplemented with a video signal in the preceding signal field by utilizing the memory unit 26 and the memory unit control circuit 31, a resultant video signal 32 may have a sag in each field, as shown in FIG. 3(g), and be fed to the D/A converter 33 for converting it into a distorted analog signal.
Accordingly, there has been a problem of sag distortion that results in a considerable disturbance of display as it is hard to detect vertical and horizontal synchronizing pulses at the synchronous separation circuit 22 in the duration of the sags.
A skew distortion has also been encountered in the prior art VTR reproducing system such shown by a block diagram in FIG. 4. Referring to FIG. 4, a reproduced FM signal 13 reproduced from a head 5 is amplified at an amplifier 12 and fed to a switcher 18 which is operated by a switching signal 17. The reproduced FM signal 19 switched at the switcher 18 is then demodulated into a video signal 43 at a demodulator 20 and clamped to a certain level at the tip end of a synchronizing signal thereof in a clamp circuit 21. The clamped video signal 44 is fed to an A/D converter 24 and is digitized therein. The digitized video signal 25 is then fed to a memory unit 26.
A memory unit control circuit 31 starts writing the digitized signal 25 into the memory unit 26 from a start point WR in the signal field and the writing and the reading are repeatedly executed at a sampling rate in the duration of the signal field whilst the video signal written in the memory unit 26 in the duration of the preceding signal field is read out during the no-signal field starting from a start point RR. The video signal 32 read out from the memory unit 26 is fed to a D/A converter 33 for converting it into an analog signal to attain a reproduced video signal 34 shown in FIG. 5 (c) (shown only by a horizontal synchronizing signal).
Since the prior art system has been arranged as described above, if the reproduced video signal performs an interlace scanning, an interval of horizontal synchronizing pulses at the switch over point will become 1.5H as it is shown in FIG. 5(c), losing a continuity of the horizontal synchronizing signal and causing such a problem as skew distortion in every field.
Moreover, a phase distortion has been encountered in a reproduced video signal in accordance with the prior art VTR reproducing system, especially in a case when a recorded video signal is reproduced in a different speed from that of the recording.
Now referring to FIG. 6, there is shown a prior art rotary drum assembly in a cross section. The rotary drum assembly consists of a rotary shaft 1, a fixed lower drum 10, an upper rotary drum 3, a head bed 4 secured to the upper rotary drum 3 by screws, video heads 5 mounted at the periphery of the head bed 4 and shown together with a video tape 2, bearings 6, an upper rotary transformer 7 fixed to the upper rotary drum 3 to be rotated therewith, a stationary lower transformer 8, a pedestal 9 secured to the rotary shaft 1 to support the upper rotary drum 3, a magnet 75 mounted on the upper rotary drum 3 and a magnetic detector 76 arranged in proximity of the magnet 75 for detecting magnetic fluxes therefrom to determine a rotary position of the upper rotary drum 3.
The video heads 5 traverse across the video tape 2 in a slant-wise by the rotation thereof and the travel of video tape 2 for forming parallel tracks as it is shown in FIG. 7. Referring now to FIG. 7(a) through (d), there are shown several tracks formed by the relative movements of the rotary heads 5 and the video tape 2, wherein A2 designates a track of the traveling video tape 2, V1 designates a normal traveling speed of the video tape 2, A5 designates tracks of the rotary heads 5 and V designates a speed of the rotary heads 5. Since the track A2 and the tracks A5 are crossed as shown in the drawing, relative tracks to be formed on the traveling video tape 2 by the rotary heads 5 are represented by tracks A in FIG. 7(a). In a still display playback mode, the traveling speed of the video tape 2 is 0 and the relative tracks A0 conform to A5. In a slow motion display playback mode, the traveling speed of the video tape 2 is decreased from V1 to Vs and the relative tracks of the rotary heads 5 against the traveling video tape 2 are represented by As in FIG. 7(b).
In a VTR employing such a rotary drum assembly, as described above, a pseudo-vertical synchronizing signal for a reproduced video signal is generated with a constant phase relationship to a head switching signal in case of playing back a recorded video signal from the video tape 2 other than the normal traveling speed of V1 and combined to the reproduced video signal to form a composite video signal. However, in this type of play-back mode, the rotary heads 5 do not trace the relative tracks A or recorded tracks, which are formed on the video tape 2 at normal traveling speed by the rotary heads 5, and a signal output level is decreased considerably, then, over and above this, noises are generated due to off tracking of the rotary heads 5. Thus, it is hard to attain a clear display from the reproduced video signal because of a poor S/N ratio.
Heretofore, it has been proposed to utilize a rotary drum assembly having an arrangement to displace the rotary heads 5 so as to trace the recorded tracks accurately under any traveling speed of the video tape 2. The rotary drum assembly of this type is shown in FIG. 8 partly in a cross section. In this illustration, a drive unit 40 displaces the rotary heads 5 in the lateral direction of the video tape 2 upon receipt of a control signal from the outside through a slip-ring coupling composed of a contact 52 and a ring electrode 53. An amount of displacement of the rotary heads 5 is so selected, for instance, in the case of slow motion play-back mode with a tape traveling speed of Vs, so as to change the tracks A5 of the rotary heads 5 to be A51 within one field play-back period of time in order to make relative tracks coincide with the relative tracks A of the normal tape traveling speed as it is shown clearly in FIG. 7(c).
More generally, it is required to displace the displaceable rotary heads by an amount of P(1-Vs/v) (where Vs/v is Vs/V1) in the duration of one field play-back period of time under any slow tape traveling speed of Vs to make the rotary heads trace the recorded tracks without deviation. If the tape traveling speed is decreased from V1 to V1/2, the same track may be traced a plurality of times by the rotary heads. In FIG. 7(d), there is shown an example wherein the video tape 2 is traveling at V1/2 and the rotary heads 5 are so displaced in the lateral direction of the video tape 2 as to align relative tracks with the relative tracks A for tracing the track A at the position of L0. In this example, a switching point of the rotary heads is given by So. In the next field, the tracing will be made on a track at the position of L1 which is parted from the previously traced track by an amount of P/2 and a switching point of the rotary heads will be given by S1 provided that the rotary heads are displaced so as to trace the track at L1. Accordingly, a phase difference between reproduced video signals derived from the tracks at the Lo and L1 is given by 1/2.multidot..alpha. H based on the switching points of So and S1.
As it has been described above, in accordance with the prior art system, there has been a problem such that if a pseudo-vertical synchronizing signal generator circuit being capable of generating a pseudo-vertical synchronizing signal with a constant phase is employed for the reproduced video signal which has a different phase in every field, there may cause vertical vibrations or a jitter in the displayed pictures due to the fact that the phase difference between the inserted pseudo-vertical signal and the reproduced video signal varies by an amount of 1/2.alpha. H for every trace (in the case of slow motion play-back with the tape traveling speed of V1/2).
It is therefore an object of the present invention to provide a VTR that can reproduce a stable image in good picture quality with no distortions by eliminating sags from the reproduced video signal.
It is an another object of the present invention to provide a VTR that can reproduce a stable image in good picture quality with no distortions by eliminating skews from the reproduced video signal.
It is still another object of present invention to provide a VTR that can produce a stable image in good picture quality with no distortions regardless of tape traveling speed by eliminating phase distortions due to use of a pseudo-vertical synchronizing signal generator being capable of generating a pseudo-vertical synchronizing signal having no phase shifting from the tape traveling speed.